1. Field of the Invention
This invention relates to methods of inhibiting and preventing the deposition of alkaline earth scale from geothermally derived brines, particularly to methods of inhibiting the precipitation and preventing the inclusion of radioactive minerals and fluoride containing minerals in scale derived from geothermal wells.
2. State of the Art
Large subterranean aquifers naturally produce steam or hot aqueous liquids, or, as used herein, "geothermal" steam or liquids. Sources of geothermal steam or liquids are found throughout the world. These aquifers, which often contain thermal energy in vast amounts, are most commonly found where the earth's near-surface thermal gradient is abnormally high. Evidence of a geothermal aquifer is unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
In some regions of the world geothermal steam or water has been used for centuries as therapy to treat physical infirmities and diseases. Geothermal steam or water has also been used to heat dwellings and for industrial processes. Although efforts to develop these site-restrictive uses of geothermal resources continue, recent research and development effort has focused on producing electrical power from geothermal resources. The electrical power produced can be conducted long distances from the geothermal resource, often over existing power grids. This is particularly advantageous, since recent steep cost increases for petroleum products used to produce conventional electric power, as well as petroleum fuel shortages or embargoes, have resulted in the desire to find an alternative, and generally self-renewing, source of power plant "fuel."
In one process for producing electrical power, a naturally pressurized, hot, substantially liquid geothermal brine at over about 400.degree. F. is flashed, reducing pressure and converting some of the brine to steam while cooling the remaining liquid geothermal brine. The steam produced in this manner then powers steam turbine generators. The cooled geothermal brine can often be used to advantage in binary systems in which a low-boiling point, secondary liquid in a closed loop is vaporized by the relatively hotter cooled geothermal brine, the vapor produced from the secondary liquid powering gas turbine generators. The cooled geothermal brine and the steam condensate obtained from power generation are typically injected into the ground to replenish the aquifer and prevent ground subsidence.
The highly saline hot brine from the wells used to generate geothermal power is a saturated or nearly saturated solution in many ions and minerals. As the brine is flashed many of these ions and minerals precipitate out. But some minerals form small particles and precipitate only slowly. If these slowly precipitating small particles of silicate containing minerals, such as iron silicate, are not forced out of solution before injection, they tend to precipitate in the injection wells, plugging them prematurely. Therefore, a flocculant must be added to the brine in settling tanks to speed the precipitations of these slowly precipitating minerals. U.S. Pat. No. 4,874,529 issued to Featherstone, the disclosure of which is hereby incorporated in full by reference, discusses the problem of fine particulates not precipitating and the use of a flocculating agent as the solution to the problem.
However, not all minerals precipitate from the brine solution slowly. Alkaline earth salts, for example, barium sulfate, also known as barite, and calcium fluoride, are among the minerals frequently found in large amounts dissolved in geothermal brines. Alkaline earth salts tend to precipitate fairly quickly. During the flashing step both barium sulfate and calcium fluoride start to precipitate out of solution, and both continue to precipitate during further handling. In many wells barium sulfate is present in concentrations high enough to start to precipitate when the brine temperature cools to as high a temperature as 340 .degree. F.
Recently it has been noticed that the crystalline material precipitated at some geothermal generation locations has become more radioactive as the location ages. This may be due to tapping into new formations containing trace amounts of the radioactive alkaline earth salt radium sulfate. Radium sulfate apparently coprecipitates with the chemically similar barium sulfate. Therefore, if barium sulfate precipitation can be inhibited, then the amount of radiation can be reduced.
At least some of the precipitate can be processed further to advantage, since it contains a high enough concentration of valuable minerals, for example, silver, to make mineral recovery, hereinafter called a "line mine," economical. U.S. Pat. No. 4,756,888 issued to Gallup et al. the disclosure of which is hereby incorporated in full by reference, teaches recovering silver in a line mine that is incorporated into a geothermal power plant and U. S. patent application Ser. No. 07/559,042 by Gallup et al., which is incorporated by reference in its entirety, teaches a process for precipitating precious metals from geothermal brines. However, calcium fluoride, the main fluoride mineral present, is present in the mined precipitate in concentrations high enough to corrode the metallurgical furnaces used for processing the precipitate. Therefore, much of these precipitates cannot be readily processed to extract the valuable minerals present.
Additives or dispersants that keep barium sulfate in solution are known. However, no dispersant has ever been used to mitigate the problems caused by calcium fluoride precipitation in geothermal systems. Moreover, the action of dispersants on barium sulfate is generally the opposite to flocculant action on siliceous materials. Clearly a method that prevents or inhibits radioactive scale and calcium fluoride from precipitating that does not interfere with the action of the flocculants used to settle slowly precipitating minerals is needed.